The present invention is generally in the field of submarine navigation. More specifically, the invention is in the field of submarine navigation using submarine launched expendable radio navigation buoy systems.
Modern submarines are equipped with “dead reckoning” (DR) navigation systems to enable them to estimate their geographic position (i.e., latitude and longitude) when submerged. Dead reckoning is a method of determining the position of a vessel via compass readings and distances traveled. Exemplary DR navigation systems include inertial navigation systems, Doppler sonar systems and speed log systems, which are used in conjunction with a compass to determine DR geographic position estimates. Due to inherent measurement inaccuracies, dead reckoning navigation systems suffer from errors that accumulate with time and distance traveled.
The inertial navigation system (INS) is an exemplary DR navigation system, which calculates geographic displacements by measuring accelerations. Inertial navigation systems are particularly useful for submarines that remain submerged for extended periods. In INS, the DR geographic position estimate of a submarine can be determined by summing INS displacement measurements relative to an accurate geographic position fix (e.g., departure port). Due to inherent inaccuracies in INS acceleration measurements, total error of the INS estimated geographic position of the submarine increases with time. To maintain adequate geographic positional accuracy, submarines must periodically acquire geographic position updates from an external source to calibrate their internal navigation systems.
An exemplary and popular known source of geographic position information is the Global Positioning System (GPS), which uses multiple orbiting satellites to provide geographic position data to GPS receivers via radio frequency (RF) signals. GPS receivers require RF signal contact with a minimum of three different satellites to obtain geographic position data. In general, the accuracy of geographic position data of a GPS receiver increases as the RF signal contact with different satellites increases. Thus, modern GPS receivers commonly have eight or more receiver channels for receiving and processing satellite RF signals from a large number of satellites. Alternate sources of RF navigation signals are also available throughout the world. An exemplary source of RF navigation signals that is operated by Russia is the Global Orbiting Navigation Satellite System (GLONASS). Another exemplary source is the Galileo System that is under development by the European Union.
Military submarines rely heavily upon stealth to be effective combat vessels in times of war and deterrents in times of peace. Surfaced (i.e., un-submerged) submarines can be easily detected visually (e.g., satellite photography) and electronically (e.g., radar). Thus, military submarines remain submerged for extended periods, during which updates of geographic position information may be required. Radio frequency signal propagation through water is greatly attenuated, and thus, receiver antennas must be above the water surface in order to receive RF signals. Typically, submerged submarines must ascend to a depth relatively close to the ocean surface to receive updated geographic position data via antennas, which they extend above the ocean surface. Disadvantageously, this process can be time consuming and is an inherently dangerous procedure. Also, surfaced or nearly surfaced submarines with extended antennas can be more easily detected than submarines at depth. Thus, methods have been developed for submerged submarines to obtain updated geographic position data while remaining submerged.
A method for submerged submarines to obtain updated geographic position data while remaining submerged is described in detail in U.S. Pat. No. 5,319,376, issued on Jun. 7, 1994 to James Eniger, which is hereby incorporated by reference in its entirety for its teachings on submarine navigation systems, submarine buoys and GPS, and is referred to hereinafter as “Eniger '376”. The method of Eniger '376 begins by releasing an arctic buoy from a submerged submarine. The arctic buoy rises until it encounters ice floating on the ocean surface. The artic buoy penetrates the ice, deploys a GPS antenna into the air above the ice surface and receives RF signals from GPS satellites. The arctic buoy transmits geographic position information to the submerged submarine via a data link such as a fiber optic or electric cable. Disadvantageously, the method of Eniger '376 does not correct for inaccuracies in geographic position information due to buoy drift (i.e., latitude and longitude displacement over time of a buoy due to ocean surface wind and current), which is normally encountered on the ocean surface. In addition, the Eniger '376 approach does not correct for submarine geographic displacement that occurs while the buoy is acquiring geographic position, which increases inaccuracies in geographic position information.
Therefore, a need exists for submarine launched expendable radio navigation buoy systems that can provide highly accurate geographic positions. Specifically, a need exists for submarine launched expendable radio navigation buoy systems that provide correction for submarine and buoy geographic displacements while the buoy is acquiring geographic position.